This invention relates to a numerical control method for a press brake or the like.
In general, a plate bending machine such as a press brake has a punch and a die. A working piece is placed on a die, and the punch is lowered to the working piece whereby the working piece is bent. In this case, it is preferable that the bending operation is carried out by using a back stopper. That is, if the bending operation is carried out after the working piece is abutted against the back stopper which is suitably positioned, then the working piece can be bent as intended.
However, sometimes the back stopper and the working piece may interfere with each other, i.e. the back stopper obstructs the bending of the working piece, in the bending operation, depending on the configuration of the working piece.
In order to overcome this difficulty, a retraction device has been proposed in the art, in which a back stopper is rotatably supported on a mounting stand, and a swinging cylinder is provided between the mounting stand and the body of a plate bending machine, so that in bending a working piece the cylinder is retracted to move the back stopper to a position where the back stopper does not interfere with the working piece.
However, the retraction device is still disadvantageous in that it is necessary to additionally provide such a swinging cylinder and a control device therefor, which costs a great deal.
Almost all the conventional numerical control devices are of the type that numerical instructions are issued by means of paper tapes and are read by a tape reader, whereby numerical control is carried out. Recently, as a result of the remarkable development of semiconductor memory devices, numerical control is carried out by storing numerical instructions is semiconductor memory devices in a numerical control device instead of paper tapes. It should be noted that, in such a system, its instruction giving method is substantially equivalent to the conventional method using paper tapes; that is, instruction data on the paper tape is merely transferred into the semiconductor memory devices. That is convenient in a sense due to the following reason: As the conventional paper tape can be utilized as memory means, as it is, it is unnecessary for the numerical control programmer to make additional studies, and the conventional paper tape can be used as it is.
However, the conventional method still involves problems to be solved with respect to operability in a certain control and the efficiency of use of memory device. For instance, in the case where a number of numerical control programs for different working pieces are stored in a memory device in one numerical control device and the numerical control programs thus stored should be selectively used, a method of classifying the programs for storing them and of selecting a desired one out of the number of numerical control programs must be provided. In the conventional method, only one program is allowed to be present in the memory device of the numerical control device. Therefore, in processing different working pieces, it is necessary to input different programs in the memory device. Accordingly, in the case where it is necessary to frequently change the kinds of working pieces to be processed, it takes a relatively long time to input the programs into the memory device. This is a serious problem.
The numerical value display unit of a conventional numerical control device is independently provided for position display only. In a recent numerical control device in which manual data input (hereinafter referred to merely as "MDI" when applicable) is effected directly from the panel, i.e. desired data are inputted directly from the panel, the display unit on the panel is used only for manual data input and cannot be used for position display.
Thus, the conventional numerical control device is disadvantageous in that two display units, i.e. the position display unit and the manual data input display unit, must be separately provided on one numerical control device as described above, and therefore the circuitry is necessarily intricate and the panel is relatively larger, which increase the manufacturing cost.
In general, in bending a plate, the rise dimension (A) is delicately affected by the difference in thickness, material and bending angle of the plate, or the working piece, the difference in bending configuration, and other various factors. Therefore, even if numerical instruction data are inputted into a numerical control device as indicated on the drawing, the dimensions of the actually finished working piece scarcely coincide with those on the drawing; that is, they include errors. In addition, it is necessary that the punch lowering distance (B) is also made somewhat different according to the above-described various factors.
Thus, in the actual press brake, it is necesary to correct the rise dimension A and the punch lowering distance B, and the distance L between the end of the punch and the back stopper, and a punch lowering distance D are defined as follows:
L=A-.alpha. PA1 D=B-.beta.
where .alpha. is the correction value for the rise dimension A, and .beta. is the correction value of the punch lowering distance B.
In a conventional numerical control device for a press brake adapted to bend a metal plate, or a working piece, a plurality of digital switches for setting a rise dimension, a punch lowering distance and correction values are provided on the panel, so that a rise dimension A, a punch lowering dimension B and correction values .alpha. and .beta. are set for every process depending on a working piece to be bent.
Such a correction method is provided in compliance with the conventional tool dimension correcting method employed in a numerical control device for machine tool, such as a numerical control device for a lathe. In the tool dimension correcting method, in order to correct the deviation of tool dimensions attributable to wear of the edge of a cutting tool, errors in setting a tool, or the like, for instance six (No. 1 through No. 6) correction value setting units are provided, and the correction value setting units are used for six hexagonal turret tools, respectively, for instance. In this case, correction can be completely achieved by setting suitable correction values for the tools.
However, in the plate bending process, the errors in dimension are not caused by the tools, but by the thickness, bending angle, bending configuration, etc. as was described before. Therefore, depending on bending processes, different correction values must be employed for different processes, and sometimes three correction values .alpha..sub.1 through .alpha..sub.3 are insufficient to conduct the bending processes.
Furthermore, the necessity for correction is not caused by the tools, but by the contents of processes. Therefore, application of the correcting method provided according to the tool dimension correcting method is not reasonable. Even if the correcting method is employed, the setting will be rather difficult.